The present invention relates to hemodialysis for removing blood-borne uremic toxins and by-products of metabolism from the blood of patients suffering from renal failure.
This procedure is performed in dialysis apparatus generally composed of one or more dialyzers in which a blood compartment is separated from a dialysate compartment by a filter structure and a dialysis machine that controls the rate of flow and composition of the dialysate and monitors the dialysis procedure.
Apparatus of this type is available in a variety of forms. For example, the apparatus may be composed of two dialyzers whose respective compartments are connected in parallel or series. Each such form of construction has advantages and disadvantages. All known arrangements have certain drawbacks that adversely affect the quality and/or speed of a complete dialysis procedure.
A typical parallel arrangement is disclosed in U.S. Pat. No. 6,117,100.
Double high flux (DHF) dialysis is a known technique that uses two high flux dialyzers in series and a volumetric controlled dialysis machine. The dialysate flow path is altered in order to provide pressure differentials across each dialyzer, causing the first dialyzer to act as an ultrafilter while the second dialyzer performs infusion of substitution fluid by backfiltration. This treatment can advantageously be performed in newer machines that offer a sufficient calibration of the transmembrane pressure (TMP) gauge and arterial pressure gauge, a high blood pump speed, two dialysate filters to filter the dialysate fluid prior to entering the dialyzer and a high pressure of the incoming water.
DHF has also been referred to as high flux hemodiafiltration. This is because this treatment combines the enhanced convective removal associated with ultrafiltration with the diffusive removal associated with counter current dialysate. The two dialyzers, in series, double the effective surface area. For example, the dialysate flow may be set to 800 ml/min, with approximately 150 ml/min of this used for backfiltration, yielding a useful dialysate flow of 650 ml/min through the first dialyzer.
There are several limitations to the current application of DHF:    1) Patient selection. Patients, for known systems DHF, must have adequate vascular blood flow in order to provide extracorporeal blood flows of 650 ml/min. Most DHF patients have fistulas, however PTFE grafts can provide adequate blood flow. The known systems and procedures can not be used with patients who either do not want larger needles for blood access or do not have a blood access that provides a good enough flow to sustain a delivered flow of at least 550 ml/min.    2) Modification of equipment: As discussed previously, several modifications to existing equipment must be made in order to enable the delivery of DHF. These add risk to the treatment and liability to the clinic that alters the equipment. However, new machines are being produced which have wider TMP and arterial pressure ranges that permit the pressures achieved during DHF.    3) If lower blood flows (<550 ml/min) are used with the existing set up of DHF, there is an increased concern of dialyzer clotting as plasma water is ultrafiltered, thereby hemoconcentrating the blood in the first dialyzer. The current therapy requires full opening of the clamp at low blood flows, which reduces the ultrafiltration capability of the system.